Aluminide coatings have been well known for a number of years and are widely used to protect metallic surfaces from oxidation and corrosion. Aluminide coatings are widely used in gas turbine engines because they are economical and because they add little to the weight of the part.
Aluminide coatings are formed by diffusing aluminum into the surface of the superalloy article to produce an aluminum-rich surface Myer. Exemplary patents showing diffusion aluminide coating processes include U.S. Pat. Nos. 3,625,750, 3,837,901, and 4,004,047. Typically, aluminide coatings are applied by a pack process. In a typical pack process, a particulate mixture, including an inert ceramic material, a source of aluminum, and a halide activating compound, is employed. The materials are well mixed and the parts to be coated are buried in the material. During the coating process an inert or reducing gas is flowed through the pack.
The pack coating process involves some complex reactions in which the halide reacts with a source of aluminum to produce an aluminum-halide vapor which circulates over the entire surface of the part. The vapor contacts the superalloy surface and decomposes, leaving the aluminum on the surface, while the halide is released to return to the aluminum source and continue the process. After the aluminum is deposited on the superalloy surface, it diffuses into the substrate. Diffusion is promoted by conducting the process at temperatures typically on the order of 1,500.degree. F. to 2,000.degree. F.
In the case of nickel-base superalloys, which am the most widely used type of superalloys, and which are used extensively in gas turbine engines, the predominant material found in the aluminide layer is NiAl which is formed near the surface. Other nickel aluminide compounds are often found further below the surface, as are compounds with aluminum and the other alloying elements found in a superalloy, including, e.g., cobalt, chromium, titanium, and refractory materials such as tungsten, tantalum, and molybdenum.
In gas turbine engines the turbine blades are invariably air-cooled to permit operation of the engine at higher temperatures. The cooling air is derived from air which is pressurized by the compressor section of the engine. As engine operating conditions increase with more modem engines, the temperature of the cooling air has gradually increased to the point where such "cooling" air may actually have temperatures as high as 1,000.degree. F. It has been observed that such high temperature cooling air causes an undesirable rate of oxidation on the internal cooling passages of the turbine blades and other air-cooled gas turbine engine hardware.
Attempts have been made to coat the surfaces of these internal passages by a so-called out-of-pack aluminide coating process as shown, for example, in the U.S. Pat. No. 4,347,267. According to this process, aluminum halide gases generated by a pack composition of the type previously described are caused to flow through the cooling passages in the turbine blade while the blade is held in a fixture. While this is a relatively successful solution to the problem, it is costly and time consuming because it usually requires a separate step in the coating process.
It is also known to fill the internal passageways in the turbine blade with a powder pack material itself and coat the internal passage surfaces during the same coating process which is used to coat the outside of the blade. This is a problematic approach inasmuch as it very difficult to completely fill the complex internal passageways in a modem gas turbine engine blade with a powder material and it is even more difficult to completely remove the powder material from the surfaces after the coating process is complete.